1. Field of the Invention
This invention relates to tin coated copper based electrical connectors having a reduced rate of copper/tin intermetallic formation. More particularly, a barrier layer containing from 20% to 40%, by weight, of nickel is disposed between the connector substrate and the coating. Alternatively, the barrier layer contains a copper/tin intermetallic compound.
2. Background of the Invention
Electrical connectors, such as sockets and plugs, are typically formed from a copper base alloy substrate that provides good electrical conductivity. When the electrical connector will be exposed to elevated temperature during operation, such as under the hood of an automobile, the substrate is formed from a copper base alloy having high strength and a resistance to stress relaxation.
The resistance to stress relaxation is recorded as a percent of stress remaining after a strip sample is preloaded to a set percent of the yield strength, typically 80%, in a cantilever mode per ASTM (American Society for Testing and Materials) specifications. The strip is then heated, typically to 125.degree. C., for a specified number of hours, typically up to 3000 hours, and retested periodically. The higher the stress remaining at each retest, the better the utility of the specified composition for spring applications.
To reduce elevated temperature tarnish of the copper base substrate and to enhance solderability, a coating layer is frequently applied to the substrate. Typical coating layers include nickel, palladium/nickel alloys, tin and tin alloys. To minimize cost, tin is frequently used.
At elevated temperatures, copper diffuses from the substrate and combines with the tin to form intermetallics such as Cu.sub.6 Sn.sub.5 and Cu.sub.3 Sn. The formation of the intermetallics reduces the amount of unreacted or free tin on the surface. This will degrade the electrical, corrosion and other performance characteristics.
It is known to interpose a barrier layer between the copper base substrate and the tin base coating layer to reduce the formation of the copper/tin intermetallic. A publication by Kay et al. appearing in the Transactions of the Institute of Metal Finishing, Volume 59, 1979, at page 169, discloses that barrier layers such as nickel, cobalt and iron, as well as alloys such as tin/nickel, copper/tin and nickel/iron, are disposed between a copper base substrate and a tin coating to reduce the formation of intermetallics.
While effective, these barriers do not provide the inhibition to intermetallic formation required for under the hood automotive applications that require a minimum amount of free tin remain after exposure to 125.degree. C. for 3000 hours.